Method and apparatus for receiving a global positioning system signal using a cellular acquisition signal

CROSS-REFERENCE TO RELATED APPLICATIONS

0001This application is a continuation-in-part of U.S. Pat. application Ser. No. 09/884,874, filed Jun. 19, 2001, now U.S. Pat. No. 6,560,534, which is a continuation-in-part of U.S. Pat. application Ser. No. 09/875,809, filed Jun. 6, 2001, now U.S. Pat. No. 6,542,820. The present Application is further a continuation-in-part of U.S. Pat. application Ser. No. 09/715,860, filed Nov. 17, 2000, now U.S. Pat. No. 6,417,301. Each of the aforementioned patent applications is herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

00021. Field of the Invention

0003The present invention relates to mobile wireless devices for mobile location systems, and more particularly to Global Positioning Satellite (GPS) receivers with improved indoor penetration for personal location systems.

00042. Description of the Related Art

0005With the advent of GPS, there has been a growing demand for mobile devices that may be used to provide a person's or an object's location. Devices built using conventional GPS receivers have been developed by a number of companies. However, these devices have significant limitations, one of which is indoor penetration.

0006To address the above limitation of conventional GPS receivers, a combination of mobile GPS receivers and cellular infrastructure communicating via wireless links has evolved. This combination of technologies, known as Assisted GPS (AGPS), combines a GPS receiver with a cellular handset. The cellular handset provides a two-way link for communicating positioning data (“aiding data”).

0007In particular, performance of a conventional GPS mobile device in indoor environments may be limited by ability of the GPS mobile device to decode a navigation data stream broadcast by each of a plurality of satellites. Among other components, each navigation data stream contains a satellite trajectory model having parameters describing a respective satellite's orbit and clock variation as a function of time. The satellite trajectory model in the navigation data stream is sometimes referred to as “broadcast ephemeris.” GPS mobile devices traditionally receive and decode the navigation data stream to extract the broadcast ephemeris, which is needed to compute position. However, a signal-to-noise ratio in indoor environments is often insufficient for navigation data bit decoding of the broadcast ephemeris. Thus, another means of ascertaining satellite orbit and clocks variations was needed.

0008In AGPS systems, the satellite orbit and clock variation, or information derived from these components, is provided to the GPS mobile device via a two-way cellular link. A two-way cellular link is used to request and receive information on such satellites, and the AGPS service is conventionally available only to authorized subscribers to a cellular network.

0009While AGPS offers improvements in indoor penetration, addition of a cellular handset and a subscription to a wireless provider adds to the cost and power consumption of a GPS receiver. Cellular handsets contain complex and costly components. For example, the cost of adding a cell phone alone to a GPS receiver may be prohibitive for GPS applications where a phone would otherwise be an unnecessary addition, let alone the addition of a subscription fee of a cellular provider. Moreover, cellular transmission consumes power.

0010Therefore, it would be desirable to provide a GPS mobile device that is comparable in cost to conventional GPS handheld devices but with the indoor penetration benefits associated with AGPS handsets.

SUMMARY OF THE INVENTION

0011The present invention provides apparatus and method for obtaining benefits associated with AGPS without requiring complete integration of a GPS device with a cellular handset. Furthermore, the present invention facilitates a GPS handheld or mobile device configured to operate without subscription to a cell phone service provider, and thus eliminates fees for such subscription. An aspect of the present invention is a GPS handheld device that comprises a cellular acquisition signal receiver or front end. It will be appreciated that circuitry required to receive an acquisition signal comprises only a portion of a complete cellular handset. Particularly, a transmitter portion for communicating with a basestation of a cellular network is not included in the GPS handheld device. Furthermore, digital signal processor and application processor(s) configured for modulating, demodulating, voice processing, call protocols, subscriber identification and the like are absent in the GPS handheld device. The cellular acquisition signal receiver allows the GPS handheld device to have an accurate time of day and/or frequency reference, thus assisting in GPS signal acquisition and GPS position computation.

0012An aspect of the present invention is a method for receiving a GPS signal. More particularly, a frequency correction burst is obtained from a cellular network. A frequency offset responsive to the frequency correction burst is determined, and a window of frequency search responsive to the frequency offset is determined for receiving the GPS signal. This may be done without having to transmit a cellular signal to the cellular network, and this may be done without having to have a subscription to the cellular network.

0013Another aspect of the present invention is a method for receiving a GPS signal to a GPS handheld device. More particularly, a time synchronization burst is obtained from a cellular network. A timing offset responsive to the time synchronization burst is determined, and a time of day responsive to the timing offset is determined for receiving the GPS signal. This may be done without having to transmit a cellular signal to the cellular network, and this may be done without having to have a subscription to the cellular network.

0014Another aspect of the present invention is a method for determining position of a GPS handheld device in proximity to a cellular basestation of a cellular network. More particularly, at least one of location information and identification information is obtained from the cellular basestation, and a position estimate of the GPS handheld device responsive to the at least one of location information and identification information is determined. This may be done without having to transmit a cellular signal to the cellular network, and this may be done without having to have fee-based access to the cellular network.

0015Another aspect of the present invention is GPS mobile device. More particularly, the GPS mobile device comprises at least one antenna. The at least one antenna is coupled to a cellular acquisition signal front end couple to receive a cellular acquisition signal. AGPS signal front end is coupled to the at least one antenna to receive a GPS signal. A GPS/cellular processor is coupled to the GPS front end and to the cellular acquisition front end. The GPS/cellular processor is configured with a GPS baseband processor in communication with the GPS front end and a cellular acquisition signal baseband processor in communication with the cellular acquisition signal front end. A reference oscillator is coupled to the GPS/cellular processor. A general purpose processor is coupled to the cellular acquisition signal baseband processor and to the GPS baseband processor, and memory is coupled to the general purpose processor.

BRIEF DESCRIPTION OF THE DRAWINGS

It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

<figref idref="DRAWINGS">FIG. 1</figref> is a network diagram of an exemplary portion of an embodiment of a GPS and cellular network in accordance with one or more aspects of the present invention.

<figref idref="DRAWINGS">FIG. 2</figref> is a network diagram of an exemplary portion of an embodiment of a GPS and computer network in accordance with one or more aspects of the present invention.

<figref idref="DRAWINGS">FIG. 3</figref> is a flow diagram of an exemplary portion of an embodiment of a mobile or handset GPS unit receiving cellular acquisition signals in accordance with one or more aspects of the present invention.

<figref idref="DRAWINGS">FIG. 4</figref> is a chip-level block diagram of an exemplary portion of an embodiment of GPS unit in accordance with one or more aspects of the present invention.

<figref idref="DRAWINGS">FIG. 5</figref> is a signal detection diagram of an exemplary embodiment of a frequency and delay window in accordance with one or more aspects of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

0022The present invention relates to implementing a GPS system using a GPS handheld configured for receiving acquisition signals from a cellular provider without requiring integration of the GPS receiver with a two-way capable cellular handset. The present invention provides one or more benefits conventionally associated AGPS while eliminating the requirement for obtaining aiding data from a cellular network.

0023An aspect of the present invention is a method for configuring the GPS device to store satellite trajectory data that replaces assistance data provided in AGPS systems. In AGPS systems, receiving assistance data from a cellular network requires a two-way capable cellular handset to request and receive such assistance data. Moreover, such services are commonly available only to paying subscribers to the cellular network. In particular, in one aspect of the present invention, satellite trajectory models are stored in memory in a GPS mobile device. The source of the satellite trajectory models stored in memory may be broadcast ephemeris, received and decoded when a GPS mobile device is outside in at least a medium signal strength signal environment in which navigation data decoding is feasible. Alternatively, satellite trajectory models may be supplied to the GPS mobile device through a computer network connection. If the satellite trajectory models comprise broadcast ephemeris, the satellite trajectory models may be valid for between approximately two and six hours. In another aspect of the invention, long-term satellite trajectory models are used, which may be valid for days. Once the satellite trajectory models are obtained and are in memory, the GPS mobile device may function indoors for the period of validity of the long-term satellite trajectory models. This eliminates the need to more frequently obtained assistance data as with conventional AGPS.

0024Another aspect of the present invention is a method for determining time offset of a GPS mobile device. More particularly, a cellular acquisition signal may comprise a time synchronization signal that is received by the GPS mobile device, enabling the GPS mobile device to establish a time of day for applying satellite trajectory data. In addition, the time synchronization signal, if sufficiently precise, may be used to establish a delay search window, decreasing the search time required for GPS signal acquisition. In addition, the time synchronization signal may be used to align coherent averaging intervals with GPS signal data bits to improve signal to noise ratio.

0025Another aspect of the present invention is a method for determining time offset of a GPS mobile device. More particularly, a cellular acquisition signal can comprise a time synchronization signal that is received at the GPS mobile device, enabling the GPS mobile device's receiver to establish a time of day for applying satellite trajectory data. In addition, the time synchronization signal, if sufficiently precise, may be used to establish a delay search window, decreasing search time required for GPS signal acquisition. In addition, the time synchronization signal may be used to align coherent averaging intervals with GPS signal data bits to improve signal-to-noise ratio.

0026Another aspect of the present invention is a GPS mobile device comprising: one or more antennas configured to receive cellular acquisition signals and GPS satellite signals; radio frequency (RF) front end circuitry for the GPS signals; RF front end circuitry for the cellular acquisition signals; a cellular acquisition signal baseband processor; a GPS signal baseband processor; a time keeping counter common to the baseband processors; a reference oscillator coupled to the time keeping counter, baseband processors and front end circuitry; a processor coupled to the baseband processors; and memory coupled to the processor. In some embodiments, the GPS mobile device may additionally comprise a computer network-docking interface or a data modem or both.

0027In the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention. However, it will be apparent to one of skill in the art that the present invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring the present invention.

0028Referring to <figref idref="DRAWINGS">FIG. 1</figref>, there is shown a network diagram of an exemplary portion of an embodiment of a GPS and cellular network in accordance with one or more aspects of the present invention. Satellite constellation <b>11</b> comprises a plurality of satellites. For purposes of illustration four satellites, namely, satellites <b>11</b>-<b>1</b>, <b>11</b>-<b>2</b>, <b>11</b>-<b>3</b> and <b>11</b>-<b>4</b>, are shown, though fewer or more satellites may be used. GPS device <b>10</b> is configured to receive one or more satellite signals <b>12</b> from satellite broadcast. GPS device <b>10</b> is configured to receive satellite broadcast signals <b>12</b> as a form of one-way communication. GPS device <b>10</b> is configured to receive one or more cellular broadcast signals <b>14</b> from cellular basestation <b>13</b>. GPS device <b>10</b> is configured to receive cellular broadcast signals <b>14</b> as a form of one-way communication. GPS device <b>10</b> may be configured to operate to receive satellite information from satellite broadcast signals <b>12</b> or a computer network connection, as described below in more detail, or both. Furthermore, especially when operating in indoor or other satellite signal-harsh environments, one or more cellular acquisition signals <b>14</b> broadcast from communication tower <b>13</b> is utilized by GPS device <b>10</b>.

0029Referring to <figref idref="DRAWINGS">FIG. 2</figref>, there is shown a network diagram of exemplary portion of an embodiment of a GPS and a computer network for obtaining satellite information, such as one or more satellite trajectory models, in accordance with one or more aspects of the present invention. GPS mobile device <b>10</b> may be put in communication with computer <b>22</b>. Computer <b>22</b> may be put in communication with computer network <b>21</b>, which may form a portion of an intranet or the Internet. Network <b>21</b> may be put in communication with server <b>23</b>. Server <b>23</b> comprises or has access to database <b>27</b>. Database <b>27</b> comprises one or more satellite trajectory models <b>39</b>, such as for respective satellites <b>11</b> of FIG. <b>1</b>. Accordingly, server <b>23</b> may be in communication with one or more GPS receiver stations <b>27</b>-<b>1</b>, <b>27</b>-<b>2</b>, and <b>27</b>-<b>3</b> via network <b>21</b> for receiving broadcast ephemeris comprising satellite trajectory models <b>39</b>. GPS mobile device <b>10</b> may have one or more satellite trajectory models <b>39</b> downloaded to it from server <b>23</b>.

0030Alternatively, server <b>23</b> may be put in communication with publicly switched telephone network (PSTN) <b>25</b> via network <b>21</b>. PSTN <b>25</b> may be put in communication with telephone <b>26</b>, which may be put in communication with GPS mobile device <b>10</b>. In this embodiment, a phone number, such as a toll free number, may be dialed in order to download one or more trajectory models to GPS mobile device <b>10</b>.

0031Connection between mobile device <b>10</b> and server <b>23</b> may be established to refresh satellite trajectory models <b>39</b>. At other times, this connection may be absent. For example, in field conditions lacking computer network <b>21</b> connectivity, GPS handheld <b>10</b> may obtain satellite information from one or more satellite signals <b>12</b> shown in FIG. <b>1</b>. Such information is typically valid for approximately two to six hours from time of broadcast. Before the validity period ends, a GPS receiver should attain another valid broadcast of ephemeris information to continue to operate.

0032In another embodiment, satellite tracking data from GPS reference stations <b>27</b>-<b>1</b>, <b>27</b>-<b>2</b>, and <b>27</b>-<b>3</b> is used in server <b>23</b> to create long-term satellite trajectory models <b>39</b>, which may be valid for periods of up to approximately one week. Orbit models and associated long-term orbit trajectory data are described in more detail in co-pending and related applications entitled “LONG TERM EPHEMERIS” to James W. LaMance, Charles Abraham and Frank van Diggelen, application Ser. No. 09/884,874, filed Jun. 19, 2001, and “METHOD AND APPARATUS FOR GENERATING AND DISTRIBUTING SATELLITE TRACKING” to James W. LaMance, Charles Abraham and Frank van Diggelen, application Ser. No. 09/875,809, filed Jun. 6, 2001. In one aspect of the present invention, long-term orbit trajectory models are used in GPS mobile device <b>10</b> to extend the period of validity of satellite trajectory models <b>39</b> provided by server <b>23</b>. This increases the interval over which GPS mobile device <b>10</b> may be used in conditions wherein computer network <b>21</b> is not readily accessible.

0033Referring to <figref idref="DRAWINGS">FIG. 3</figref>, there is shown a flow diagram of an exemplary portion of an embodiment of a mobile or handset GPS unit receiving cellular acquisition signals in accordance with one or more aspects of the present invention. Cellular basestation <b>13</b> broadcasts several cellular acquisition signals including frequency correction signal <b>31</b>, time synchronization signal <b>32</b>, timing message, such as a frame number, signal <b>33</b>, and cell identification number signal <b>34</b>A. Notably, time message signal <b>33</b> may be a separate signal or may be a time message <b>33</b> provided with time synchronization signal <b>32</b>. In some embodiments of cellular basestation <b>13</b>, an additional signal, namely cell location signal <b>34</b>B, is provided. It should be understood that one or more of these broadcast elements <b>31</b>, <b>32</b>, <b>33</b>, <b>34</b>A and <b>34</b>B may be present or absent in particular cellular network implementations. Furthermore, in some cellular networks one or more of broadcast elements <b>31</b>, <b>32</b>, <b>33</b>, <b>34</b>A and <b>34</b>B may be combined into various combinations of composite signals. In accordance with one or more aspects of the present invention, one or more of these signals <b>31</b>, <b>32</b>, <b>33</b>, <b>34</b>A and <b>34</b>B may be utilized, whether individually, jointly, or in various combinations.

0034Conventionally, cellular acquisition signals are provided to enable, at least in part, a cellular handset to synchronize to a cellular basestation, as a first step in establishing communication with a cellular network. In particular, in the first phase of establishing communication, the cellular handset monitors specific frequencies for the acquisition signals. In accordance with one or more aspects of the present invention one or more cellular acquisition signal is received, but a GPS mobile device does not continue with the subsequent steps needed to establish two-way communication with the cellular network. In particular, in accordance with one or more aspects of the present invention, a GPS mobile device does not transmit any data or message or both to the cellular network. Furthermore, a GPS mobile device may receive one or more acquisition signals without a cellular network subscription.

0035Referring to <figref idref="DRAWINGS">FIG. 4</figref>, there is shown a block diagram of an exemplary portion of an embodiment of a mobile or handheld GPS <b>10</b> in accordance with one or more aspects of the present invention. With continuing reference to FIG. <b>4</b> and additional reference to <figref idref="DRAWINGS">FIG. 3</figref>, extra circuitry is added to a conventional GPS receiver to allow one or more cellular acquisition signals <b>31</b>, <b>32</b>, <b>33</b>, <b>34</b>A and <b>34</b>B, collectively and singly cellular acquisition signals <b>102</b>, to be received. This includes a second radio frequency (RF) tuner, namely cellular acquisition front end <b>131</b> coupled to an additional antenna, namely antenna <b>111</b>. One or more cellular acquisition signals <b>102</b> are received by antenna <b>111</b> and provided from cellular acquisition front end <b>131</b> to cellular acquisition signal baseband <b>136</b>. Cellular acquisition signal baseband <b>136</b> is used to lock and decode one or more cellular acquisition signals <b>102</b>, for example, using conventional digital processing well known in the design of cell phones. For cost considerations, cellular acquisition front end <b>131</b> may be integrated into a conventional GPS front end <b>132</b>, which is coupled to GPS antenna <b>133</b>, on a single RF semiconductor integrated circuit <b>130</b>. Moreover, to save cost, cellular acquisition signal baseband <b>136</b> may be integrated with a conventional GPS baseband <b>137</b> on a single digital signal processing semiconductor integrated circuit to provide a GPS/cellular processor <b>135</b>. Examples of such integrated circuits include, but are not limited to, a digital signal processor (DSP). However, more than one integrated circuit may be used, for example, a DSP and an application specific integrated circuit (ASIC), and a DSP and an FPGA. Accordingly, by providing integrated circuits <b>130</b> and <b>135</b>, only a marginal increment in cost is added to a conventional GPS. Furthermore, other technologies such as radio frequency CMOS (complimentary-metal-oxide-semiconductor) allow integration of baseband processor functions and front end functions into a single ASIC. Included in this marginal incremental cost are additional filters and an extra antenna, described in more detail with respect to FIG. <b>4</b>. Moreover, because cellular acquisition signals <b>102</b> are relatively high in power, a simple antenna may be used in order to control costs even further. Alternatively, a single antenna <b>100</b> capable of receiving both GPS and cellular signals may be employed.

0036The nature of a frequency correction signal <b>31</b> varies depending on the cellular network. In CDMA systems, frequency correction signal <b>31</b> may comprise a pilot channel. The pilot channel is a common channel that is broadcast over a cell coverage area. Conventionally, the pilot channel uses a repeating pseudonoise (PN) sequence of 2¹⁵chips. Multiple basestations transmit the same PN code but at different timing offsets to avoid mutual interference. To detect the pilot channel, GPS mobile device <b>10</b> may scan a range of PN code offsets until energy is detected, indicating a cellular basestation transmitter, using a cellular acquisition front end <b>131</b> of FIG. <b>4</b>. By phase or frequency locking to a detected pilot signal, GPS mobile device <b>10</b> may measure a frequency offset <b>35</b>A related to error in a reference oscillator of GPS mobile device <b>10</b>, such as reference oscillator <b>138</b> of FIG. <b>4</b>. Alternatively, GPS mobile device <b>10</b> may make an open loop measurement of frequency error of a pilot signal to determine frequency offset <b>35</b>A.

0037In GSM systems, frequency correction signal <b>31</b> is a frequency correction burst transmitted periodically by a basestation on one of several frequency channels assigned to the basestation. The frequency correction burst signal <b>31</b> is an unmodulated tone transmitted at a specific offset from a carrier frequency of the channel. GPS mobile device <b>10</b> may make an open loop measurement of frequency error from the frequency correction burst to determine frequency offset <b>35</b>A.

0038As one or more of broadcast signals <b>31</b>, <b>32</b>, <b>33</b>, <b>34</b>A and <b>34</b>B are well-known, for example in General System Mobile (GSM) systems and Code Domain Multiple Access (CDMA) systems, among other known cellular systems, unnecessary details regarding such signals are not repeated here for purposes of clarity.

0039In an alternative embodiment, optional reference oscillator steering circuit <b>142</b> of <figref idref="DRAWINGS">FIG. 4</figref> is used to provide reference oscillator steering <b>35</b>B. In this embodiment, frequency correction signal <b>31</b> is used in connection with steering circuit <b>142</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) to maintain reference oscillator <b>138</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) within its nominal operating frequency range. Steering circuit <b>142</b> may comprise a digital-to-analog converter connected to a voltage control input of an oscillator <b>138</b> (shown in FIG. <b>4</b>). Frequency steering a reference oscillator based on a frequency correction signal is well known in cellular handsets and is generally a requirement to ensure that the handset transmitter is precisely maintained at an assigned transmission frequency. However, as GPS mobile device <b>10</b> does not comprise a cellular signal transmitter, there is no requirement to maintain a transmission frequency. Furthermore, GPS devices, in contrast to AGPS devices, tend to operate off a non-steered reference oscillator, such as an oscillator without voltage control. Thus, a non-steered reference oscillator <b>138</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) may be used along with frequency correction signal <b>31</b> to provide, or compute such as with cellular acquisition baseband processor <b>136</b> (shown in FIG. <b>4</b>), frequency offset <b>35</b>A. Frequency offset <b>35</b>A is provided to frequency and delay search window <b>41</b>.

0040An example of a frequency and delay search window <b>45</b> for a particular GPS satellite signal <b>11</b> is shown in FIG. <b>5</b>. As will be understood to those familiar with the art, frequency and delay window <b>45</b> comprises a two dimensional space of uncertain frequency on frequency window <b>501</b> axis and uncertain code delay on delay window <b>502</b> axis. To acquire a GPS signal, a GPS receiver <b>10</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) searches frequency and delay search windows <b>501</b> and <b>502</b>, respectively for a GPS signal. An exemplary GPS signal response <b>503</b> is shown in FIG. <b>5</b>. GPS receiver <b>10</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) detects signal response <b>503</b> by scanning with one or more search bins <b>504</b>. If frequency and/or delay uncertainty is large, this search can be time consuming. This is especially true in indoor environments where, in order to obtain needed signal-to-noise ratio enhancements, GPS receiver <b>10</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) dwells for periods of several seconds accumulating signal power before advancing search bin <b>504</b>. Thus, it is beneficial to keep frequency and delay search windows <b>501</b> and <b>502</b> as small as possible, especially for indoor operation.

0041With continuing reference to FIG. <b>5</b> and renewed reference to <figref idref="DRAWINGS">FIG. 1</figref>, frequency window <b>501</b> is a function of Doppler uncertainty (due to the relative motion of GPS device <b>10</b> with respect to GPS satellite <b>11</b>) as well as frequency uncertainty due to imprecision of reference oscillator <b>138</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) in GPS device <b>10</b>. Frequency offset <b>35</b>A (shown in <figref idref="DRAWINGS">FIG. 3</figref>) provides an accurate estimate of offset from a reference oscillator <b>138</b> (shown in <figref idref="DRAWINGS">FIG. 4</figref>) as frequency correction signal <b>31</b> is conventionally transmitted at a precise frequency. Thus, contribution of reference oscillator <b>138</b> uncertainty to the frequency window <b>501</b> may be substantially reduced or eliminated with frequency offset <b>35</b>A. It should be noted that adjustment of frequency window <b>501</b> may occur using a software algorithm in program memory <b>145</b> (shown in FIG. <b>4</b>), with no special purpose circuit for steering voltage or controlling frequency of reference oscillator <b>138</b>. Alternatively, offsetting of frequency window <b>501</b> may be achieved by altering frequency of reference oscillator <b>138</b> with reference oscillator steering circuit <b>142</b> (shown in FIG. <b>4</b>).

0042With renewed reference to <figref idref="DRAWINGS">FIG. 3</figref>, time synchronization signal <b>32</b> may be obtained by GPS mobile device <b>10</b> to determine time offset <b>36</b>. The nature of time synchronization signal <b>32</b> varies depending on the cellular network. In CDMA systems, time synchronization signal <b>32</b> can comprise a synchronization channel. The synchronization channel is a common channel that is broadcast over a cell coverage area. The pilot channel and synchronization channel of a particular cellular basestation use an identical PN sequence, such as a PN sequence of 2¹⁵chips. Additionally, the synchronization channel is modulated with a particular Walsh code, allowing it to be separated from paging and traffic channels using different Walsh codes. The synchronization channel carries a timing message <b>33</b>. Specifically, in CDMA, the synchronization channel carries a message containing a pilot PN offset that identifies time of day offset of such pilot channel.

0043With renewed reference to <figref idref="DRAWINGS">FIGS. 3 and 4</figref>, in a CDMA compatible embodiment, GPS device <b>10</b> may first detect a pilot channel of a nearby basestation <b>13</b> with cellular acquisition front end <b>131</b>, then proceed to decode a synchronization channel being broadcast by the same basestation <b>13</b>. GPS device <b>10</b> achieves synchronization to such a pilot channel at a particular timing offset of GPS device <b>10</b> local timekeeping counter <b>139</b>. Shortly thereafter, GPS device <b>10</b> receives time message <b>33</b> containing a time of day offset of such a pilot channel. GPS device <b>10</b> uses time message <b>33</b>, along with the timing offset of local timekeeping counter <b>139</b>, to compute time offset <b>36</b>. Since, in CDMA, basestation <b>13</b> time of day is synchronized to GPS time used by GPS satellites <b>11</b> (shown in FIG. <b>1</b>), time offset <b>36</b> provides an absolute offset between local timekeeping counter <b>139</b> and GPS time.

0044In GSM systems, time synchronization signal <b>32</b> is a synchronization burst transmitted periodically by basestation <b>13</b> on one of several frequency slots assigned to cellular basestation <b>13</b>. Time synchronization signal <b>32</b> contains a unique header, such as a known sequence of bits, that identifies a starting point of a burst. In addition, time synchronization signal <b>32</b> carries a timing message <b>33</b> that comprises, among other elements, a GSM time stamp associated with such synchronization burst. In a GSM compatible embodiment, GPS device <b>10</b> receives time synchronization signal <b>32</b>, and uses header information therefrom to identify a starting point of a synchronization burst therein relative to local timekeeping counter <b>139</b>. GPS device <b>10</b> uses this information, combined with timing message <b>33</b>, to compute time offset <b>36</b>. In this manner time offset <b>36</b> provides an offset between local timekeeping counter <b>139</b> and GSM timing of basestation <b>13</b>. In some GSM networks, GSM timing is not synchronized with GPS time. Therefore, time offset <b>36</b> does not provide an absolute time offset to GPS time. Time offset <b>36</b> may however be used beneficially as an indicator of relative time, as discussed below.

0045Time accuracy of time offset <b>36</b> will be dependent on the cellular network implementation. In systems such as CDMA that incorporate GPS timing within the cellular network, there is a high degree of timing accuracy. In other networks, for example GSM, relative timing of a time synchronization burst may be good, but an unknown offset may exist to GPS time. Finally in some systems, a time indicator may be an absolute indicator, but with limited accuracy, for example time coming from a computer server <b>23</b> (shown in FIG. <b>2</b>), in which time and date were manually set.

0046Depending on accuracy, available time offset <b>36</b> may be employed for several purposes within GPS device <b>10</b>. If timing offset <b>36</b> has precision substantially better than one millisecond, precise time component <b>41</b> of timing offset <b>36</b> may be incorporated into frequency and delay search window <b>45</b>. Specifically, with additional reference to <figref idref="DRAWINGS">FIG. 5</figref>, it is well known that, in the general case when precise timing is not available, delay window <b>502</b> spans an entire period of C/A code, nominally one millisecond (C/A code conventionally refers to codes available for civilian applications). This is because timing of locally generated C/A code within GPS baseband processor <b>137</b> is arbitrary relative to GPS signals <b>12</b> (shown in FIG. <b>1</b>). However, if precise time component <b>41</b> is available, locally generated C/A code can be timed relative to GPS signals <b>12</b> (shown in FIG. <b>1</b>). Specifically, GPS device <b>10</b> uses timekeeping counter <b>139</b> that is common to a C/A code generator within GPS baseband <b>137</b> and to cellular acquisition baseband <b>136</b>. Thus, time offset <b>36</b>, determined from a time synchronization signal <b>32</b> as described above, can be used in conjunction with local timekeeping counter <b>139</b> to program a starting point of locally generated code relative to GPS timing. In this manner, an uncertainty component of delay window <b>502</b> caused by an unknown relative timing of locally generated code is substantially reduced or eliminated. A remaining delay window <b>502</b> component is delay uncertainty related to unknown pseudorange and any error in precise time component <b>41</b>. As discussed below, a pseudorange may be estimated from satellite trajectory models and an estimate of position, such as a position information <b>34</b><i>b</i>. Thus, if precise time component <b>41</b> is accurate to substantially less than one millisecond, delay window <b>502</b> may be reduced to substantially less than one millisecond.

0047With continuing reference to <figref idref="DRAWINGS">FIGS. 3</figref>, <b>4</b> and <b>5</b>, reducing frequency window <b>501</b> as an aspect and delay window <b>502</b> as another aspect, substantially reduces the total number of search bins needed to overall cover two dimensional frequency and delay search window <b>45</b>. As mentioned, this enables GPS receiver <b>10</b>, or more particularly GPS baseband processor <b>137</b>, to search more rapidly, and therefore reduce the time needed to obtain GPS satellite signals <b>12</b> (shown in FIG. <b>1</b>). Furthermore, a reduced search window provides GPS mobile device <b>10</b> an opportunity to dwell longer at each search bin. Longer dwells provide signal-to-noise ratio enhancements that can enable weak signal reception indoors.

0048Optionally, time offset <b>36</b> is provided to coherent averaging <b>50</b>. Coherent averaging <b>50</b> improves signal-to-noise ratio in each search bin by averaging correlation results from several consecutive cycles of C/A code. When coherent averaging, impact of 50 bps navigation data bits on a GPS signal is to be considered. Specifically, due to navigation data bits, a GPS signal undergoes a potential 180 degree phase transition every 20 cycles of C/A code. For signal-to-noise ratio enhancement, coherent averaging is performed over twenty consecutive cycles of C/A code comprising a single navigation data bit. Furthermore, to enhance performance this averaging process should be synchronous with navigation data bit timing, otherwise changing data bits may partially defeat such an averaging process. For this reason, it is desirable to achieve synchronization of coherent averaging <b>50</b> with navigation data bit timing. Navigation data bit timing is uniform for all satellites <b>11</b> (shown in <figref idref="DRAWINGS">FIG. 1</figref>) and synchronized with GPS time.

0049Timing of a data bit arriving at GPS device <b>10</b> is a function of local timekeeping counter <b>139</b> as well as pseudorange delay between GPS device <b>10</b> and a satellite <b>11</b> (shown in FIG. <b>1</b>). Precise time component <b>41</b> establishes a relationship between local timekeeping counter <b>139</b> and GPS time. Thus, if pseudorange is estimated as described below, precise time component <b>41</b> may be used in conjunction with local timekeeping counter <b>139</b> to control start and stop times of coherent averaging <b>50</b> so as to make a coherent averaging interval coincident with incoming navigation data bits.

0050Time of day <b>42</b> is the absolute component of time offset <b>36</b>, converted to units of GPS time units. This conversion can take several forms, for example, a conversion from a Julian data system, or some other timekeeping standard employed by the cellular network. Time of day <b>42</b> may be utilized within GPS receiver <b>10</b> even in an application where precision of time offset <b>36</b> is not better than one millisecond, namely, when precise time component <b>41</b> cannot be generated. In particular, time of day <b>42</b> provides a reference time for ascertaining satellite positions <b>43</b>. Specifically, time of day <b>42</b> provides a reference time for satellite trajectory model <b>39</b>. As satellites <b>11</b> move rapidly through the sky, it is preferable that time of day <b>42</b> be accurate, within approximately at least ten milliseconds, so that errors in prediction of satellite positions will be on the order of meters or less. If, however, time of day <b>42</b> does not provide this level of accuracy, error in time of day <b>42</b> may be solved for as part of a navigation solution. In the latter situation, accuracy of time of day <b>42</b> is unimportant, and a rough estimate of time is sufficient such as the time provided by a server or a real time clock. An example of such a method is Time Free GPS, as described in more detail in co-pending application entitle “METHOD AND APPARATUS FOR TIME-FREE PROCESSING OF GPS SIGNALS” to Frank van Diggelen, application Ser. No. 09/715,860, filed Nov. 17, 2000.

0051If time offset <b>36</b> has an arbitrary relationship to GPS time, time of day <b>42</b> will not be directly available. However, time offset <b>36</b> may be beneficial as an indicator of relative time. For example, GPS mobile device <b>10</b> may determine an initial time of day by the conventional method of decoding the time of week (TOW) portion of a navigation data stream. TOW can be used to determine a relationship between cellular network time and GPS time. For example, time offset <b>36</b> may represent offset between GSM system time and GPS time of day. Once this relationship is established, it may remain constant for extended time periods as cellular basestations use precise oscillators to generate their timing signals. Thus, GPS mobile device <b>10</b> may use time offset <b>36</b> to determine time of day <b>42</b> based on a previously determined relationship between a cellular network and GPS time. In this manner, GPS mobile device <b>10</b> may be able to obtain positions in indoor operating environments, utilizing a time synchronization burst from time synchronization signal <b>32</b> to ascertain time of day. Furthermore, GPS mobile device <b>10</b> can function without a battery powered real-time clock to maintain time.

0052Cellular base station <b>13</b> may provide a cell identification number <b>34</b>A. The details of this message vary with cellular network. Cell identification number <b>34</b>A may be used to look up location of cellular basestation <b>13</b> in a lookup table <b>37</b> stored in memory of GPS mobile device <b>10</b>. This will give an approximate or estimated position <b>38</b> of GPS mobile device <b>10</b>, namely, GPS mobile device <b>10</b> will be within the sector associated with longitude and latitude of communication tower <b>13</b> location. As sector sizes vary from rural, suburban and metropolitan area networks, this position estimate <b>38</b> will vary accordingly depending on location of cellular basestation <b>13</b> within one of the above-mentioned area networks. In those instances where cellular base station <b>13</b> is configured to provide its cell location <b>35</b>B, cell location lookup table <b>37</b> may be avoided and an estimate of position <b>38</b> provided based on cell location signal <b>34</b>B.

0053Estimate of position <b>38</b> is provided for line of sight calculation <b>40</b>. Specifically, estimate of position <b>38</b> is combined with satellite positions, velocities and clock estimates <b>43</b> to determine expected pseudoranges and pseudorange rates <b>44</b>, and unit vectors <b>49</b> between GPS device <b>10</b> and each GPS satellite <b>11</b> (shown in FIG. <b>1</b>). Line of sight calculation <b>40</b>, pseudorange and pseudorange rates <b>44</b>, unit vectors <b>49</b>, and delay and frequency measurements <b>47</b>, are sufficient for position, velocity, and time computation <b>48</b>. The details of such computations are well known and will not be repeated here for purposes of clarity.

0054Pseudo range and pseudorange rates <b>44</b> are provided to frequency and delay search window <b>45</b>. In particular, pseudorange rate provides an estimate of Doppler shift between GPS mobile device <b>10</b> and each GPS satellite (shown in FIG. <b>1</b>), allowing frequency window <b>501</b> to be determined. Similarly, pseudorange provides an estimate of timing delay between GPS mobile device <b>10</b> and each GPS satellite <b>11</b> (shown in FIG. <b>1</b>), facilitating determination of delay window <b>502</b>. As mentioned above, pseudorange and pseudorange rate <b>44</b> are components of frequency and delay search window <b>45</b>, more particularly frequency uncertainty in reference oscillator <b>138</b> and time uncertainty of locally generated C/A code tied to time keeping counter <b>139</b>, both of which may be substantially reduced by means of cellular acquisition signals <b>102</b>.

0055Integrated circuit <b>135</b> may comprise a time keeping counter <b>139</b> for providing clock signals to baseband processors <b>136</b> and <b>137</b>. A reference oscillator <b>138</b> may be used to provide a determined frequency within a tolerance to timekeeping counter <b>139</b>. A general purpose processor, such as a microprocessor, <b>141</b> is coupled to receive information from acquisition signal baseband <b>136</b> to provide an output to GPS baseband <b>137</b>, as described with reference to FIG. <b>3</b>. Microprocessor <b>141</b> is coupled to memory <b>146</b>, which may comprise partitioned memory or individual memories <b>144</b> and <b>145</b>. For individual memories, program memory <b>145</b> is used to store programming, as described with reference to <figref idref="DRAWINGS">FIG. 3</figref>, for using one or more cellular acquisition signals to provide information regarding satellite range and range rate. Accordingly, program memory may be a programmable, non-volatile memory, such as an EPROM, E²PROM, flash memory, and the like. Data memory <b>144</b> may be used to temporarily store data for microprocessor <b>141</b>. Accordingly, data memory <b>144</b> may be a programmable volatile memory, such as DRAM, SRAM, and the like. Optionally, a docking station, data modem and/or network interface <b>143</b> may be coupled to microprocessor <b>141</b> for receiving one or more satellite trajectory models. Optionally, a digital-to-analog (D/A) converter <b>142</b> may be coupled to microprocessor <b>141</b> to receive a digital signal of a frequency and convert it to an analog signal of the same frequency for providing a steering voltage to reference oscillator <b>138</b>.

0056It should be appreciated that the incremental circuitry in the GPS device to receive and utilize cellular acquisition signals is minimal. In particular, the scope and cost of this circuitry is far less than that of a complete cell phone, which would include transmission circuitry, digital signal processing circuitry, voice processing circuitry, a protocol stack processor, and many other components. Thus, it is anticipated that a GPS system in accordance with one or more aspects of the present invention may be manufactured with less cost than that to produce conventional AGPS system.

0057While the embodiments described herein have provided details for GSM and CDMA systems, it should be apparent that the invention can be employed in all types of cellular networks including iDEN, TDMA, AMPS, GPRS, CDMA-2000 and other 2.5 networks, and W-CDMA and other 3G networks. Furthermore, the invention can accept multiple types of cellular acquisition signals in a single device. In particular cellular acquisition front end <b>131</b> and cellular acquisition baseband <b>136</b> may be configured to incorporate simultaneous or sequential processing of signals from multiple networks. This would further facilitate use of a GPS device <b>10</b> anywhere in the world, not just within a prescribed coverage region, and accordingly it would be desirable to provide an ability to receive and use a set of cellular network signals.

0058Though GPS satellites were described, it should be appreciated that one or more aspects of the present invention may be used with pseudolites, ground based transmitters that broadcast a PN code similar to a GPS signal. Accordingly, the term “satellites”, as used herein, is intended to include pseudolites and equivalents thereof. Moreover, the term “satellite signals” or “GPS signals” is intended to include satellite-like and GPS-like signals from pseudolites and equivalents thereof. Furthermore, though a GPS system was described, it should be appreciated that one or more aspects of the present invention are equally applicable to similar satellite positioning systems, including without limitation the Russian Glonass system.

0059While the foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.